Secondary Flame Burn Types and Related Stove Top Heat

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Woodcutter Tom

Feeling the Heat
Apr 28, 2019
255
Northern Illinois
I would like to start a discussion on secondary burning flame types and their relationship to stove top heat. I would like to confine this to tube stoves and not stoves with catalytic converters. Just to keep on the same topic and not be confusing to anyone.

My goal at the end of this is to learn which secondary flame types provide the best stove top heat. And I want to understand how to create fires that create those flame types.

As I see it there are 4 types of secondary flames. I’ll list them as:

Type 1. Slow lazy light blue clouds of flame that dance in midair. May not be continuous; may just appear then vanish. Very little primary flames in the firebox.

Type 2. Light orange flames dancing. They look gassy and dance about. Sometimes mixed with blue flames and particles can be seen igniting.

Type 3. Heavy orange gassy flames. Usually filling the entire firebox. Associated with a high heat reading on the flue thermometer.

Type 4. Shooting flames from multiple secondary burn tube holes. Intense and strong. Sometimes extending across to the next tube. Causes the baffle that rests on the tubes to become red from heat. Look like mini blow torches.

Keep in mind that I do not see these as ‘set-in stone’ types. They change and blend as the fire constantly changes. Please feel to define what you have experienced.

If the goal is stove top heat, what type of secondary flames does a user want to see? And how does a person I get that type?
 

firefighterjake

Minister of Fire
Jul 22, 2008
19,593
Unity/Bangor, Maine
Well I've heard folks describe four types as well . . . just in bit different terms.

Fireworks: Little to no flames and then a burst of flame at the top one third of the firebox . . . followed a few seconds later with another burst. I tended to notice this type of secondary burn when my wood was less than optimally seasoned and/or when I cut the air back too much as I suspect this is a case where the fire is more or less smoldering, producing combustible gases which slowly collect and then reach a critical mass at which point they ignite.

Ghost Flames: The orangey-pale flames slowly wave back and forth above the baffle, almost in slow motion. I see these often, especially early in the secondary burn.

Propane BBQ: The blue jets of flame coming out of the secondary burn tubes. In fairness, I have never seen these with my woodstove.

Portal to Hell: Almost the entire firebox is consumed with a fiery inferno and your first thought is "Oh no. I've finally done it. I have opened a Portal to Hell and Beelzebub." I get this a lot . . . especially with very well seasoned wood.

Honestly, I have never really thought which type of secondary burn is the best . . . all I know is I am generating lots of heat in the home, I get decent burns (recently I've been getting 9 hour burns where I load the stove, get the secondary established and go to bed only to wake up many hours later to marble-sized coals which can easily relight small splits or kindling) and that my chimney is nice and clean.

I suspect the most optimal burn would be the Ghost Flames -- enough air to keep a continued secondary without generating so much excess heat that a lot goes up the flue . . . but honestly that would be a pure, speculative guess on my part.
 
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EbS-P

Minister of Fire
Jan 19, 2019
3,571
SE North Carolina
So I think there not a real easy answer but I will lend some observations and thoughts.
Flame color depends on temperature and the amount of unburned particulates. Blue is hotter, yellow is colder



To that end when I burned bio bricks the the secondary flames were blue land purple and my even with the air full closed my stove top temps were 600+.

when I burn poplar they are more yellow and orange and stove top is 500-550. Then towards the end of the burn they get more of the glassy blue.

the gates of hell secondaries happen when I get rapid out gassing caused by small splits of not turning down air soon enough. The. Cut air way back. Color depends on temps. Cold start more yellow hot reload more blue.

efficiency is different than max heat output. I think the most efficient way to run a stove is to slow the flue gasses down as much as possible, giving more time to release heat to the stove/room. At the same time having enough heat in the firebox for complete combustion. So based on that I think the best flame is which ever one is continuous at the lowest possible air setting is best.

Need more heat. Open the air control speed up combustion higher temps faster flue gasses. Higher stove top temps but more energy up the flue.

just my thoughts.
Evan
 

Woodcutter Tom

Feeling the Heat
Apr 28, 2019
255
Northern Illinois
Thanks for your input guys. I had thought that there would be more interest in this subject. Oh well.

I am trying to learn how to get heat out of my stove. That is the whole reason for having a stove. I have had the different types of secondary flames, but with this stove I have not had the really forceful Propane blue jets. I have never had over 550 degrees STT. I have never been 'forced from the room' from heat. I use seasoned walnut, ash and hickory. The hickory seems to give off more gasses. The walnut lights quicker.

Yesterday I added back 2 feet of chimney which I had removed when I went from a 1.2 cu ft stove to a 1.9 cu ft. I'm now at 17 feet above stove top. I'll see how that goes. Today a smaller load in the lincoln log fashion brought the STT up to 450 much quicker than a full load I did yesterday. Maybe there is something to be learned there. Anyway, thanks for your input.
 

brazilbl

Burning Hunk
Aug 24, 2017
124
El Dorado County, CA
Having a “small firebox” as well, I see very similar results in terms of STT and waking up to pebble sized coals after 9 hours overnight. If need be, I can get the house past 80. I remember working on a train table one night and feeling sweat. Then I knew this stove can produce...
 

Ctwoodtick

Minister of Fire
Jun 5, 2015
1,782
Southeast CT
I suspect a lot of the variation probably has to do a lot with brand/model of stove. Ive been Burningwood for a while and I don’t think I’ll be able to identify how exactly I achieve each type of secondary. Certainly, the inferno from hell type scenario can be easily achieved by leaving the primary way open for way too long. Not advisable.
 

Das Jugghead

Burning Hunk
Jan 2, 2019
160
Indiana
I have secondaries every time I load the stove. I mostly burn ash, walnut, cherry, and a few pieces of oak and mulberry. With a small EPA stove and a small firebox such as mine there is no sense in burning softwoods.

My secondaries start as vigorous orange or yellow flames streaming out of the holes on the secondary tubes and then become blue/purple. Ultimately they will be replaced by blue or purple ghost haze flames that hover above the wood. My favorite secondary action to watch is when the ghost flames become "pop-up thunderstorms" . They seem to disappear and then the entire top of the firebox ignites in a beautiful sudden eruption of purple flame that often originates from one point and then rapidly traverses the entire ceiling of the firebox before disappearing again - this can go on for an hour. Ultimately this is replaced by an orange column of flame about an inch or so in Diameter that comes from somewhere within the wood and travels straight up to the ceiling of the firebox. It is almost as if a column of liquid flame is being poured upside down from the wood to the top of the firebox.
 

jatoxico

Minister of Fire
Aug 8, 2011
4,366
Long Island NY
Flame color depends on temperature and the amount of unburned particulates. Blue is hotter, yellow is colder
efficiency is different than max heat output.

I tend to look at it this way too. A full firebox of orange sooty flame may be hot but not terribly efficient. I try to keep my eye on flame color and shoot for bright yellow or better, adjusting air accordingly.
 

firenovice

New Member
Dec 4, 2020
2
Grafton, MA
I’m always fighting to keep the blue bbq - keep it between smoldering and inferno. I’m starting to dial in the air control based on species to keep the blue bbq going in the tubes. I seem to always have a few yellow/orange columns on the side connecting coals to the bbq. I don’t have a name for the connector flames yet.
 

begreen

Mooderator
Staff member
Nov 18, 2005
96,455
South Puget Sound, WA
Yesterday I got the billowy floating secondaries that I love. They were orange and blue. I get these more when burning hardwood, which is rare for me, but last night there was some madrona and wild cherry in the mix.

This is a lively secondary burn in a Castine.
 
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Highbeam

Minister of Fire
Dec 28, 2006
20,016
Mt. Rainier Foothills, WA
There’s two goals that seem to be being mixed up. Do you want maximum stove top heat or do you want maximum efficiency which might occur at much lower output levels?

People don’t get glowing red stoves burning slow.
 
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kennyp2339

Minister of Fire
Feb 16, 2014
6,707
07462
To understand secondary's you have to know when they form (1100def f) Any wood stove can create secondary flames but its the epa tube stoves that essentially harness the power for clean burn and additional heat. The air tubes usually have there own air supply which works in-conjunction with the primary fire, the hotter the primary fire, the more vigorous the secondary's due to more air being pulled into the secondary manifold.
The secondary air tubes act as turbulence enhancers, the air that is supplied to them gets heated by the primary fire, then exits out the tube injecting fresh oxygen into the top of the fire box which then creates another point of oxygen rich air (which adds to replace the burnt oxygen from the primary fire) and it mixes with the unburnt particles from the primary fire and re-burns them
I been through the 4 different types of secondary's myself and feel as though the best stove top heat is usually when the base of the fire has low whispy flames and the secondary's provide a gas range oven like burn, gentle blues w/ oranges.
Certainly wood type, split size and dryness plays a big part in this, can't burn water and softer woods cant create an extended period burn times like a full load of dry white oak, smaller splits create many voids within the firebox which relates to more wood face exposure to off gas faster.
 

bigealta

Minister of Fire
May 22, 2010
863
Utah & NJ
begreen thanks for the video. i'm always striving for those secondaries at the baffle tubes that are "above" and separated from the wood below. But they are pretty elusive. The f400seems so touchy. I'm burning oak splits and when i start reducing the draft below halfway even in tiny reductions, i seem to often snuff out the flames. Then i need to open up the draft again and basically start over the draft reducing progression. Looking for a better way to get and Hold the secondaries for as long as possible.
Also Kennyp thanks for your detailed post about secondary action.
 

bigealta

Minister of Fire
May 22, 2010
863
Utah & NJ
Here is a short video of the secondary flames and how they can end. This is in a Jotul f400 castine. Hope this helps anyone who's interested.
 

Das Jugghead

Burning Hunk
Jan 2, 2019
160
Indiana
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